Table 4 Proposed moderators and mediators

 Growth velocity_w0

Offset from predicted age at peak height velocity in years, calculated from clinically measured anthropometrics: sitting height, bodyweight, total stature, biological age [51]. Participants will be categorized into pre- (<− 1.0 years offset), circa- (− 1.0 to + 0.5 years offset), and post-PHV (> + 0.5 years offset), corrected for timing of the primary endpoint, resulting in a 5 month subtraction.

Overuse knee injuries, as well as injuries to the growth plate, in sports active adolescents are higher during peak height velocity and the year leading up to this point [52,53,54,55,56], which is thought to be primarily due to vulnerable growth-related conditions, such as Osgood-Schlatter [52]. This has been supported by data showing higher growth velocity for Osgood-Schlatter patients than controls [57].

 Growth timing_w0

Based on algorithms for calculating anticipated age of peak height velocity which incorporates data on parents adult stature, we will classify participants as early maturers, average maturers, and late maturers (girls: < 10.94, 10.94–12.94, and > 12.94 years, respectively; boys: < 12.64, 12.64–14.64, and > 14.64 years, respectively) [58, 59].

Reaching skeletal maturity either late or on average is a risk factor for developing Osgood-Schlatter compared to early maturation [2, 60].

 Fear of movement_w13

Fear of movement will be captured by participants filling in the 17 items Tampa Scale of Kinesiophobia, each item scored on a 4-point likert scale with a score ranging from 0 to 68 [61, 62].

Avoidance behavior might be related to a too apprehensive approach to gradual exposure and exercise therapy resulting in the patient not achieving a higher degree of participation in sports and physical activity. Osgood-Schlatter patients exhibits a high degree of kinesiophobia [44].

 Pain intensity_w0

Self-reported “worst pain past week” on the 0–10 NPRS.

Pain intensity has shown to be related to a worse prognosis for adolescents with anterior knee pain [63]

 Pain frequency_w0

Self-reported on the P1 question of the KOOS-child Pain subscale on frequency of experience knee pain, answered on a 0–4 ordinal scale ranging from “Never” to “All the time” [43].

Pain frequency have shown to be related to a worse prognosis for adolescents with anterior knee pain [63, 64].

 Treatment expectations_w0

Self-reported through the question on change in “my ability to self-manage my knee pain” answered on a 1–4 likert scale from ‘worse’, ‘no difference’, ‘little improvement’, to ‘large improvement’.

Treatment expectations have been shown to moderate outcomes in trials of several different musculoskeletal conditions and chronic pain conditions [65,66,67,68,69].

 Tibial tubercle maturation_w0

Rated by sonographers ad modum Sailly [70, 71] on a 1–4 scale depending on features of cartilage, potential secondary ossification center, tendon, and the infrapatellar bursa.

The level of maturation of the apophysis has been shown to be related to the prevalence of Osgood-Schlatter symptoms with early (exhibiting no metaphysis-physis junction or apophyseal attachment of the patella tendon) and late stages (denoting full unification of metaphysis-physis junction and matured attached of the patella tendon) being low-risk stages, and the intermediate stages (exhibiting open metaphysis-physis junction, apophyseal attachment of the patella tendon, active ossification center) have a higher association to symptoms [70, 72, 73]

 Severity_w0

Rated by sonographers ad modum Flaviis [73] from ‘cartilage attachment’ to ‘mature attachment’ on a 1–4 scale.

Severity on the Flaviis scale has been associated with a worse prognosis [9, 73].